The present invention relates to an intraocular lens which is to be implanted in place of the natural eye's crystalline lens that has been removed in surgical operations on cataract and so on.
In surgical operations on cataracts, the crystalline lens is removed from the natural eye and replaced by an artifical crystalline lens. Conventionally, intraocular lenses made of polymethyl methacrylate resins (hereinafter referred to as PMMA resins), which are used for the surgical operations, are widespread in the U.S. and many other countries in the world.
Such an intraocular lens is implanted in a location that is close to the place where the natural eye's crystalline lens was positioned so that the state of an image focused on the retina is very close to that attained by the normal eye. Accordingly, the intraocular lens is particularly suitable for use as artificial crystalline lenses from the viewpoint of the restoration of the patient's vision after operation. Also, the intraocular lenses require no daily care and are preferred for aged people who are frequent victims of cataract.
The application of intraocular lenses made of PMMA resins is increasing today and further improvements in their properties such as biocompatibility and transparency are needed.
Conventional intraocular lenses made of PMMA resins have very high degrees of light transmission in the visible range and biocompatibility and have been used in clinical applications. However, they suffer from the disadvantage that they are transparent not only to visible light but also to light in the ultraviolet and near ultraviolet (260-350 nm) regions.
FIG. 3 shows the transmittance characteristics of a prior art intraocular lens made of PMMA resin. As shown in FIG. 3, the conventional intraocular lens transmits ultraviolet rays at wavelengths below 250 nm and its transmittance of light in the near ultraviolet range longer than 250 nm is sharply increased to at least 90%. If such as intraocular lens is implanted in a patient's eye, his eyeball, in particular, the optic nerve may be damaged by ultraviolet radiation. There also is a problem in association with visual sensation; that is, the eye fitted with this lens which transmits light in the ultraviolet and near ultraviolet regions produces a more bluish sensation than the normal eyeball.
In order to solve these problems, the incorporation of uv absorbers and/or pigments in PMMA resins has been attempted but it has turned out that these additives change or reduce the biocompatibility of the intraocular lens, with some of them being particularly undesirable since they are potentially carcinogenic.